Rapid rotation of energy in the excited state of a circular hexa-carbazole array
Introduction
Since the discovery of circular chlorophyllic arrays within the photosynthetic machinery of certain light-harvesting bacteria 1, 2there has been intense interest in the mechanism of energy storage and rotary diffusion within such antenna complexes 3, 4, 5, 6, 7, 8, 9. This interest arises from a curiosity in the workings of natural photosynthesis itself and from the possibility that similar molecular complexes could be synthesised in the laboratory and their unique properties utilised in man-made solar energy conversion devices. In the present work we have synthesised a simple model compound for such circular chromophoric arrays with the intention of investigating whether its photophysical properties resemble those of the much more complex natural systems. We have found that this is in fact the case with rapid, rotary diffusion of energy occurring in a time of 3 ps via a transient, dipolar excimer-like state. Our molecule consists of a circular array of six carbazole moieties arranged around a benzene core, in a propeller configuration as shown in Fig. 1.
Section snippets
Experimental
Hexacarbazolylbenzene (HCB) was prepared by an adaptation of the synthetic procedure reported for hexapyrrolylbenzene [10], using the sodium anion of carbazole and hexafluorobenzene. Because of the poor solubility of HCB the dodeca-hexyl substituted derivative, DDH–HCB, was synthesised and used in most of the measurements. Where comparison was possible the optical properties of HCB and DDH–HCB were found to be almost identical after correction for monomer impurity in the former, less soluble
Results and discussion
The optical absorption and emission spectra of the hexacarbazole derivative DDH–HCB and the reference monocarbazole compound MCB are shown in Fig. 2. The most pronounced difference in optical properties between the two compounds is seen to be the shape and solvent sensitivity of the fluorescence. Thus, the vibrationally structured and weakly solvent dependent emission of MCB, with a maximum at 343 nm in cyclohexane, is replaced for DDH–HCB by an emission with a maximum at 394 nm which is broad
Acknowledgements
The present investigation was supported by The Netherlands Organisation for the Advancement of Research (NWO).
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